A fuel cell utilizes the chemical energy of a fuel to produce directly electrical energy. This electrochemical conversion of fuel has a higher electrical efficiency than conventional energy generation, by eliminating mechanical losses. In addition to economic benefits, direct electrochemical conversion results in significant environmental advantages, by reducing emissions of greenhouse gas and eliminating emissions of toxic pollutants.
One of the several types of the fuel cells is a solid oxide fuel cell (SOFC). Solid oxide fuel cells are a promising technology for efficient and environmentally friendly power generation. The single cell of the SOFC possesses three basic parts: an electrolyte that conducts oxygen ions, an anode that produces electrons, and a cathode that consumes electrons. The most common fuel in an SOFC is synthesis gas which is produced from any fossil or prepared fuel and which consists mainly of hydrogen and carbon monoxide. Using synthesis gas as the fuel fed to the anode and oxygen gas from ambient air as the oxidant fed to the cathode, the following reactions occur: at the anode: 2H.sub.2+20.sup.2−=2H20+4e.sup.− and 2C0+20.sup.2−.=.2CO.sub.2+4e.sup−; at the cathode: O.sub.2+4e.sup=20.sup.2−; for the cell as a whole: 2H.sub.2+0.sub.2=2H.sub.20+heat and 2C0+20.sup.2−.=.2CO.sub.2+heat.
A conventional SOFC utilizes a yttria stabilized zirconia (YSZ) ceramic for an electrolyte. The anode is a Ni-YSZ-cermet. Lanthanum-strontium manganate (LSM) is used for the cathode material.
The voltage produced by a single cell is about one volt. To increase voltage, the cells may be combined electrically in series to create a stack. The following materials are utilized for stacking the cells: an interconnect to provide electrical connection of the cells and a seal to provide mechanical gas-tight connection of the cells. Various non-scaling alloys and lanthanum-strontium chromate are usually used for the interconnect.
SOFCs may be constructed in a variety of geometries. A planar cell, such as a flat plate construction may have a self supporting anode, cathode or electrolyte sheet onto which the other components are mounted.
Planar fuel cell stacks can be constructed using a cross-flow configuration with external manifolds as exemplified for instance by U.S. Pat. No. 4,950,562 issued Aug. 21, 1990 and No. 5,856,035 issued Jan. 5, 1999. Gas channels are formed due to a set of grooves in the interconnect surface. This design provides uniform gas flow distribution within the cell's electrodes because the manifolds are very wide. However the design does not provide uniform gas flow distribution between the cells in the stack, because the cells are connected in parallel in respect of the manifolds. In this design, a considerable part of both the electrolyte and the interconnect surfaces are utilized for sealing and thereby decreasing the efficiency of material utilization.
Planar fuel cell stacks may also be constructed using a co-flow or counter-flow configuration with internal manifolds as exemplified for instance by U.S. Pat. No. 5,227,256 issued Jul. 13, 1993 and No. 6,824,910 issued Nov. 30, 2004. In this design, gas channels are formed by a set of grooves in the interconnect surface. However gas flows are not quite uniform within the cell's electrodes and especially between the cells in the stack because the cells are connected in parallel in respect of the manifolds which are very narrow. As in the abovementioned design, a considerable part of the surface both of the electrolyte and especially of the interconnect are utilized for sealing and therefore not utilized for power production, thereby decreasing the efficiency of material utilization.
In the abovementioned designs, both the cell and interconnect are flat. A planar design where the cell is not flat is exemplified by the German Pat. No. 25 14 034 issued Aug. 17, 1978. The support structure of the cell is a wave-like electrolyte and the interconnect is flat. The gas channels are formed by the wave-like cell surface in contact with the flat interconnect. In this design, the interconnect area is less than the cell working area, the interconnect has no grooves and therefore is thin, thus decreasing the stack material intensity. However, the stack structure is not durable because the electrolyte membrane is under mechanical stress from bending. The main problem with this design is providing gas tight separation of fuel and oxidant flows at the cell inlet.